Liquid condensates (by-products of gas production) and some crude oils are known to contain numerous metallic trace compounds often in the form of organometallic complexes. These metallic compounds can frequently poison the catalysts used during transformation of these fractions into commercial products.
Purification of feedstocks for use in transformation processes for condensates or crudes is thus advantageous in order to avoid arsenic entrainment. Purification of the feedstock upstream of the treatment processes protects the installation assembly.
Some of the applicants' processes perform well as regards mercury or arsenic removal from liquid hydrocarbons used as feedstock for various treatment processes. U.S. patent U.S. Pat. No. 4,911,825 clearly demonstrates the advantage of mercury and possibly arsenic retention in a two step process wherein the first step consists in bringing the feedstock in the presence of hydrogen into contact with a catalyst containing at least one metal from the group constituted by nickel, cobalt, iron and palladium. Mercury is not (or is only slightly) retained by the catalyst but it is retained, in a second step, by a bed comprising sulfur or sulfur compounds.
Patent application WO 90/10 684 from the applicant describes a process for elimination of mercury and if necessary arsenic present in liquid hydrocarbons. This invention concerns catalysts having the ability to resist sulfur poisoning (thioresistance). These novel catalysts allow mercury and arsenic to be retained under conditions which are too severe for the catalysts described in the prior art.
This process is particularly useful in the purification of difficult feedstocks such as, for example, gas oils from fractionation of crude oil whose sulfur content is frequently between 0.4 and 1.0% by weight. On the other hand, the process described in U.S. patent U.S. Pat. No. 4,911,825 is more effective for feedstocks with a lower sulfur content, for example less than 0.15% by weight.
It has been established, however, that with some feedstocks having a low sulphur content, for example less than 0.07% by weight, the arsenic retention efficiency at the beginning of the arsenic removal process is lower in the first hundreds of hours than later on. It has also been found that the arsenic retention efficiency is lower for feedstocks with very low sulphur contents, for example less than 0.02% by weight. In the latter case, it is necessary to increase the operating temperature of the reactor by several dozen degrees and/or increase the hydrogen flowrate to retain sufficient arsenic.
U.S. patent U.S. Pat. No. 4,046,674 describes an arsenic elimination process (for quantities greater than 2 ppm) using a retention bed containing at least one nickel compound (comprising at least one sulphide) in quantities of 30-70% by weight NiO, and at least one molybdenum compound (comprising at least one sulphide) in quantities of 2-20% by weight MoO.sub.3. This mixed sulphide absorbant requires the presence of large quantities of sulfur (greater than 0.1%) in the feedstock and high operating temperatures (of the order of 288.degree. C. and 343.degree. C. in the examples) in order to avoid desulfurization.
The present invention overcomes these drawbacks.